Starter/generator subsystems for small engine applications typically utilize DC brushed motor/generators and an electronic Generator Control Unit (GCU). The DC brushed machine has a number of disadvantages in comparison to a permanent magnet machine of equivalent capacity, including increased size and weight, significantly lower reliability and higher maintenance. The DC brushed machine requires relatively frequent maintenance, including replacement and servicing of brushes and bearings. Typically, maintenance intervals are 600 to 1000 operating hours for brushed machines in small aircraft applications. The brushed starter/generator also typically exhibits significant reductions in generator capability when operating at low speeds, and small aircraft applications often require operation over relatively wide speed ranges (typically 50% to 100% operational range in the generator mode).
Currently, permanent magnet generators (PMGs) are sometimes used as starter/generator subsystems for small engine applications. However, the PMG starter/generator subsystems of the current art have several disadvantages. Since the output voltage and frequency of the PMG varies directly with driveshaft speed, a more complicated regulation and start excitation control method is required, as compared to that of a brushed starter/generator. Also, the PMG output voltage cannot be controlled by means of adjusting stator field excitation (which is a common control technique used with brushed starter/generators), and therefore the PMG output cannot be electronically disabled in the event of a fault. The inability to disable PMG output power during fault mode operation is of particular concern in high reliability applications such as aircraft. A feeder cable short to engine or aircraft structure can result in a hazardous or unsafe condition for typical PMG/Power Conversion Unit (PCU) architectures. A series contactor or circuit breaker is typically required to disable the PMG output voltage. However, the PMG is typically located on or near the engine system and must survive an extremely harsh environment, particularly in aircraft applications. That harsh environment makes it very difficult (and often not technically feasible) to locate a contactor or circuit breaker close to the PMG.
U.S. Pat. No. 5,929,537 describes a starter/generator subsystem utilizing a PMG and bi-directional PCU. However, the starter/generator architecture as described and illustrated in this patent is a non-isolated topology PMG and non-isolated topology PCU. Therefore, very few if any of the “fault tolerant” aspects are achieved. The U.S. Pat. No. 5,929,537 also describes an architecture requiring contactors to isolate the PCU, battery, and/or AC loads. The use of high power contactors to isolate these subsystems is necessary for practical non-isolated architectures since a single break-down in the feeder cable to aircraft structure can result in a catastrophic failure condition. The PCU described in the U.S. Pat. No. 5,929,537 does not provide several reliability and/or performance features, including power factor corrected generator mode operation; ability to detect ground faults within the PMG, PCU and/or feeder cables; failsafe operation capability including operation in the presence of shorted windings in the PMG, shorted feeder wires, and/or failed PCU converters; or soft start and/or motoring start capability.
U.S. Pat. Nos. 5,594,322 and 5,493,200 describe a generator design with a field winding on the rotor that receives field current for control of the generator output. The PMG described in these patents is entirely different than the PMG design described for this invention. The subsystems described in the U.S. Pat. Nos. 5,594,322 and 5,493,200 also do not incorporate the fault tolerant capabilities described for the subsystem described by this invention.